Alternating pump gaps

ABSTRACT

A blood pump system includes a pump housing and an impeller for rotating in a pump chamber within the housing. The impeller has a first side and a second side opposite the first side. The system includes a stator having drive coils for applying a torque to the impeller and at least one bearing mechanism for suspending the impeller within the pump chamber. The system includes a position control mechanism for moving the impeller in an axial direction within the pump chamber to adjust a size of a first gap and a size of a second gap, thereby controlling a washout rate at each of the first gap and the second gap. The first gap is defined by a distance between the first side and the housing and the second gap is defined by a distance between the second side and the pump housing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/041,987, filed Feb. 11, 2016 and entitled“ALTERNATING PUMP GAPS,” which claims priority to U.S. ProvisionalApplication No. 62/115,318, filed Feb. 12, 2015 and entitled“ALTERNATING PUMP GAPS,” which is hereby incorporated by reference inits entirety.

BACKGROUND OF THE INVENTION

The present invention relates in general to pumping devices, and morespecifically, to improved blood pumps with levitated impellers andmethods for their control.

Mechanical circulatory support (MCS) devices are commonly used fortreating patients with heart failure. One exemplary type of MCS deviceis a centrifugal flow blood pump. Many types of circulatory supportdevices are available for either short term or long term support forpatients having cardiovascular disease. For example, a heart pump systemknown as a left ventricular assist device (LVAD) can provide long termpatient support with an implantable pump associated with anexternally-worn pump control unit and batteries. The LVAD improvescirculation throughout the body by assisting the left side of the heartin pumping blood. Examples of LVAD systems are the DuraHeart® LVASsystem made by Terumo Heart, Inc. of Ann Arbor, Mich. and the HeartMateII™ and HeartMate III™ systems made by Thoratec Corporation ofPleasanton, Calif. These systems typically employ a centrifugal pumpwith a magnetically levitated impeller to pump blood from the leftventricle to the aorta. The impeller is formed as the rotor of theelectric motor and rotated by the rotating magnetic field from amultiphase stator such as a brushless DC motor (BLDC). The impeller isrotated to provide sufficient blood flow through the pump to thepatient's systemic circulation.

Early LVAD systems utilized mechanical bearings such as ball-and-cupbearings. More recent LVADs employ non-contact bearings which levitatethe impeller using hydrodynamic and/or magnetic forces. In one example,the impeller is levitated by the combination of hydrodynamic and passivemagnetic forces.

There is a trend for making blood pumps more miniaturized to treat abroader patient population, more reliable, and with improved outcomes.To follow this trend, contactless impeller suspension technology hasbeen developed in several pump designs. The principle of this technologyis to levitate the pump impeller using one or a combination of forcesfrom electromagnets, hydrodynamics, and permanent magnets. In themeanwhile, the pump should be hemocompatible to minimize the blood celldamage and blood clot formation. To that end, the bearing gap betweenthe levitated impeller and the pump housing becomes an important factor.A small gap may lead to the high probability of the thrombus formationin the bearing or to elevated hemolysis due to excessive shear stress.Likewise, a large gap can compromise the hydrodynamic bearingperformance and the pump efficiency.

One pump design utilizing active magnetic bearings achieves the desiredbearing gap by levitating the impeller using magnetic fields generatedby electromagnetic coils. However, in such a design there is the needfor a separate bearing control system that includes the position sensorsand electromagnetic coils to control the impeller position.

Another pump design levitates the impeller using hydrodynamic thrustbearings alone or combined with passive magnetic bearings. However, sucha design usually requires a small bearing gap to provide sufficienthydrodynamic bearing stiffness to maintain impeller levitation andprevent contacts between impeller and the pump housing. Such a small gapmay result in an insufficient washout and vulnerability to bloodclotting thus compromising hemocompatibility.

Pumps utilizing hydrodynamic bearings to suspend the impeller aregenerally designed to maintain a generally constant gap through alloperating conditions. A drawback of such designs is that the impellerstarts to tilt when the pump flow rate increases. This impeller positionshift under low pressure conditions across the narrow gap creates bloodflow stasis, which in turn leads to thrombus formation on the bearingsurfaces. One solution to solve the problem is to add a passive magneticbearing to try to maintain a stable gap. However, the magnetic bearingsolution increases the size of the pump and complexity of the system.

What is needed is a pump that addresses these and other problems ofknown designs. What is needed is a pump with a relatively small formfactor and improved outcomes. What is needed is a pump that employs asimple bearing system and enhances blood flow gaps to reduce the risk ofthrombus. What is needed is a solution to enhance the bearing gap toachieve adequate washout without increasing the complexity of the pumpmechanical design or reducing the pump efficiency.

BRIEF SUMMARY OF THE INVENTION

In summary, various aspects of the present invention are directed to ablood pump system including a pump housing; an impeller for rotating ina pump chamber within the housing; a stator comprising drive coils forapplying a torque to the impeller; a bearing mechanism for suspendingthe impeller within the pump chamber; and a position control mechanism.

Various aspects of the invention are directed to a blood pump systemincluding a pump housing; an impeller for rotating in a pump chamberwithin the housing; a stator comprising drive coils for applying atorque to the impeller; a first bearing for fixing the impeller relativeto a first end of the pump chamber, a first blood gap defined betweenthe impeller and a first bearing surface; a second bearing for fixingthe impeller relative to a second end of the pump chamber, a secondblood gap defined between the impeller and a second bearing surface; anda position control mechanism.

In various embodiments, the position control mechanism is configured toalternate the pump secondary flow gaps. In various embodiments, theposition control mechanism is configured to move the impeller in anaxial direction within the pump chamber to adjust a blood gap distancebetween the impeller and an opposing wall of the housing. In variousembodiments, the position control mechanism is configured to move theimpeller in an axial direction within the pump chamber to increase thefirst blood gap thereby increasing a washout rate.

In various embodiments, the washout rate is the average washout rateduring a respective period of time. In various embodiments, the washoutrate is the peak washout rate during a respective period of time. Forexample, the respective period of time may be the period during whichthe impeller is moved to a target position to increase the washout rate.

In various embodiments, the position control mechanism is configured tomove the impeller periodically and intermittently. The position controlmechanism may be configured to move the impeller for at least severalseconds every minute. The position control mechanism may be configuredto move the impeller based on a triggering event. The position controlmechanism may be configured to move the impeller based on the impellercrossing a speed threshold. The speed threshold may be a low speedthreshold. The triggering event may be based on a low speed thresholdand time threshold.

In various embodiments, the pump is configured with at least a firstbalanced position with a narrow first gap and a second balanced positionwith a narrow second gap. The impeller may be controlled such that theimpeller spends substantially equal amounts of time in the first andsecond balanced positions. In various embodiments, the amount of timethe impeller spends in each balanced position is inversely proportionalto the gap size. In various embodiments, one of the gaps is identifiedas being prone to stasis and the impeller spends more time in a positionaway from the identified gap.

In various embodiments, the movement of the impeller is asynchronouswith the native heartbeat. In various embodiments, the movement of theimpeller is synchronous with the native heartbeat.

In various embodiments, a total blood gap under normal operatingconditions is 50 micrometers. In various embodiments, a total blood gapunder normal operating conditions is 100 micrometers. In variousembodiments, a total blood gap under normal operating conditions is 200micrometers. In various embodiments, a total blood gap under normaloperating conditions is 1000 micrometers. In various embodiments, atotal blood gap under normal operating conditions is 2000 micrometers.In various embodiments, the impeller is moved to a position to decreasea respective blood gap by about 20%, by about 30%, by about 40%, byabout 50%, by about 60%, by about 70%, by about 75%, by about 80%, or byabout 90%.

Various aspects of the invention are directed to a method of operating apump as described in any of the preceding paragraphs.

Various aspects of the invention are directed to a method of operating ablood pump including a housing and an impeller for rotating within apump chamber within the housing including rotating the impeller withinthe pump chamber, the impeller being suspended within the pump chamberby a first bearing at a first end of the pump chamber and a secondbearing at a second end of the pump chamber; and moving the impeller inan axial direction within the pump chamber to increase a first blood gapdefined by the first bearing and to decrease a second blood gap definedby the second bearing.

Various aspects of the invention are directed to at least one system,method, or computer-program product as described in the specificationand/or shown in any of the drawings.

In one aspect, a blood pump system is provided. The pump system mayinclude a pump housing and an impeller for rotating in a pump chamberwithin the housing. The impeller may have a first side and a second sideopposite the first side. The pump system may also include a statorhaving drive coils for applying a torque to the impeller and at leastone bearing mechanism for suspending the impeller within the pumpchamber. The pump system may further include a position controlmechanism for moving the impeller in an axial direction within the pumpchamber to adjust a size of a first gap and a size of a second gap,thereby controlling a washout rate at each of the first gap and thesecond gap. The first gap may be defined by a distance between the firstside and the housing and the second gap is defined by a distance betweenthe second side and the pump housing.

In another aspect, a blood pump system may include a pump housing, animpeller for rotating in a pump chamber within the housing, and a statorhaving drive coils for applying a torque to the impeller. The pumpsystem may also include a first bearing for fixing the impeller relativeto a first end of the pump chamber. A first blood gap may be definedbetween the impeller and a first bearing surface. The pump system mayfurther include a second bearing for fixing the impeller relative to asecond end of the pump chamber. A second blood gap may be definedbetween the impeller and a second bearing surface. The pump system mayinclude a position control mechanism for moving the impeller in an axialdirection within the pump chamber to increase the first blood gapthereby increasing a washout rate at the first blood gap.

In another aspect, a method is provided of operating a blood pumpincluding a housing and an impeller for rotating within a pump chamberwithin the housing. The method may include rotating the impeller withinthe pump chamber. The impeller may be suspended within the pump chamberby a first bearing at a first end of the pump chamber and a secondbearing at a second end of the pump chamber. The method may also includemoving the impeller in an axial direction within the pump chamber toincrease a first blood gap defined by the first bearing and to decreasea second blood gap defined by the second bearing.

The structures and methods of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated in and forma part of this specification, and the following Detailed Description ofthe Invention, which together serve to explain the principles of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an implantable pump as one example of a rotarymachine employing the present invention.

FIG. 2 is an exploded, perspective view of the exemplary centrifugalpump of FIG. 1.

FIG. 3 is a cross-sectional view of the exemplary pump of FIG. 1,illustrating the impeller levitated at a first balanced positiongenerally centered within the pumping chamber in accordance with aspectsof the invention.

FIG. 4a is a schematic view of the exemplary pump of FIG. 1,illustrating the impeller levitated eccentrically in the pump chamber bythe main bearing components.

FIG. 4b is a schematic view of the pump of FIG. 1, illustrating theimpeller moved to another eccentric position at the bottom of the pumpchamber.

FIG. 4c is a schematic view of the pump of FIG. 1, illustrating theimpeller moved to yet another eccentric position at the top of the pumpchamber.

FIG. 5 is a flowchart showing a method of controlling impeller positionin accordance with the invention.

FIG. 6 is a block diagram of a pump control system in accordance withthe invention.

FIG. 7 is a line chart depicting the method of controlling the impellerposition in accordance with the invention.

FIG. 8 is a flowchart showing a method of controlling impeller positionin accordance with the invention.

FIG. 9 is a flowchart showing a method of moving the impeller inaccordance with the invention.

FIG. 10 is a flowchart showing a method of starting up a pump inaccordance with the invention.

FIG. 11 is a cross-sectional view of a centrifugal flow pump inaccordance with aspects of the invention, illustrating a supplementalelectromagnetic bearing.

FIG. 12 is a cross-sectional view of an axial flow pump in accordancewith aspects of the invention, the axial flow pump including mechanicalbearings.

FIG. 13 is a perspective view of the impeller of FIG. 12, with arrowsdepicting the direction of translation in accordance with the invention.

FIG. 14 is a cross-sectional view of an axial flow pump in accordancewith aspects of the invention.

FIG. 15 is a cross-sectional view of an axial flow pump in accordancewith aspects of the invention, the axial flow pump includinghydrodynamic and magnetic bearings.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to those embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims.

For convenience in explanation and accurate definition in the appendedclaims, the terms “up” or “upper”, “down” or “lower”, “inside” and“outside” are used to describe features of the present invention withreference to the positions of such features as displayed in the figures.

In many respects the modifications of the various figures resemble thoseof preceding modifications and the same reference numerals followed bysubscripts “a”, “b”, “c”, and “d” designate corresponding parts.

As used herein, “gap” generally refers to the secondary flow gaps aroundthe impeller as would be understood by one of skill in the art. Theprimary flow is through the impeller blade regions. The secondary flowgaps are the other areas of fluid, generally around the impeller. Insome respects, the secondary flow gaps are between the impeller and thehousing wall and define the hydrodynamic bearing.

The term “machine-readable medium” includes, but is not limited toportable or fixed storage devices, optical storage devices, wirelesschannels and various other mediums capable of storing, containing orcarrying instructions and/or data. A code segment or machine-executableinstructions may represent a procedure, a function, a subprogram, aprogram, a routine, a subroutine, a module, a software package, a class,or any combination of instructions, data structures, or programstatements. A code segment may be coupled to another code segment or ahardware circuit by passing and/or receiving information, data,arguments, parameters, or memory contents. Information, arguments,parameters, data, etc. may be passed, forwarded, or transmitted via anysuitable means including memory sharing, message passing, token passing,network transmission, etc.

Furthermore, embodiments of the invention may be implemented, at leastin part, either manually or automatically. Manual or automaticimplementations may be executed, or at least assisted, through the useof machines, hardware, software, firmware, middleware, microcode,hardware description languages, or any combination thereof. Whenimplemented in software, firmware, middleware or microcode, the programcode or code segments to perform the necessary tasks may be stored in amachine readable medium. One or more processors may perform thenecessary tasks.

Although aspects of the invention will be described with reference to ablood pump, one will appreciate that the invention can be applied toother types of pumps. The mechanisms and methods of the invention willbe described in relation to blood pumps and in particular the ability toadjust the impeller operating position to address performance, such asthe attendant risks for thrombus and hemolysis when pumping blood. Onewill appreciate from the description herein that the invention can beapplied broadly to other pumps, rotary machines, and induction motors.

Turning to the drawings, aspects of the invention enable to the abilityto enhance or control the bearing gaps. One might wish to increase thebearing gap to adjust the washout rate, lubricate the bearing surfaces,or remove materials (particulates, thrombus, etc.) from the bearing gap.Another use of the invention may be to increase pump efficiency. As isknown in the art, the motor efficiency increases as the impeller magnetmoves closer to the motor drive coils. Another use of the invention maybe to correct impeller malpositioning due to bulk forces or externalforces (e.g. bumps or movements of the patient's body). These and otheradvantages can be achieved without the need for complex control systemsin accordance with the invention.

Various aspects of the invention are directed to improving the washoutin a respective pump bearing gap by moving the impeller to a position toincrease the respective gap size. The impeller may be moved periodically(e.g. by time) or triggered by an event. When the impeller moves up, thelarger gap on the bottom of the pump leads to a higher flow rate, whichin turn leads to a higher washout rate. The upward movement of theimpeller may also “squeeze” the blood above the impeller in a sort ofpumping action, which also increases the washout rate. The fluid aboveis squeezed as the impeller moves, but once the impeller is in the newposition the pumping effect is lost whereas the higher washout ratestill remains below at the larger gap. In other words, the pumpingeffect on washout rate occurs at a specific point in time whereas thelarger gap effect is temporal in nature. The basic concept makes use ofthe fact that the gap size is correlated to the washout rate. Thewashout rate relates to an average time for a full exchange of fluid.Accordingly, if the gap size divided by the washout rate is equal to afew seconds that doesn't necessarily mean the impeller should move foronly a few seconds. Oftentimes, the impeller is moved for between about100%, 120%, 150%, 200%, or other percentage of the average time for fullexchange calculated by the gap size divided by the washout rate to havehigher confidence. The washout flow rate is generally proportional tothe cube of the gap width. Therefore, theoretically the total washoutflow rate will be 8 times greater when the gap becomes 2 times wider.See W. K. Chan et al., Analytical Investigation of Leakage Flow I DiskClearance of Magnetic Suspended Centrifugal Impeller, Artificial Organ(2000). Washout rate is vital, as a failure to get full washout mayresult in thrombosis formation in areas of stasis or where fluid isn'texchanged. This creates two risks: 1) the thrombus may dislodge and flowinto the body, thus causing an embolism (or a stroke depending on whereit goes), and 2) the thrombus may continue unabated, causing theactivated site to form more platelets, which in turn provide a site forthrombin to adhere.

Accordingly, alternating the gaps has been found to improve thehydrodynamic bearing washout while maintaining a small total gap (goodpump efficiency, proper HD bearing operations, etc.). In other words,alternating the gaps improves the washout without increasing the totalgap.

The above technique can be implemented in a pump with a hydrodynamicand/or electromagnetic bearing. With a hydrodynamic bearing design, itis expected that there will be power loss associated with movement ofthe impeller. In one embodiment using electromagnets and hydrodynamicpressure grooves, the power loss can be minimized by designing thebearing stiffness curve to have at least two stable (eccentric)positions and facilitating movement of the impeller between these atleast two positions. In one example, the electromagnets need additionalforce only to move the impeller from one side to another, and no suchforce is required to keep the impeller on one side.

In one embodiment, an electromagnetic force control method is used tochange the impeller position and enhance the effective gap between theimpeller and the blood chamber. The technique uses the same pump motorstator coils adjust the impeller position as is used to apply a torqueto the impeller. No additional control subsystems and components arenecessary.

Turning now to the drawings, wherein like components are designated bylike reference numerals throughout the various figures, attention isdirected to FIG. 1 which depicts an exemplary pump implanted in a heartfailure patient.

A typical cardiac assist system includes a pumping unit, driveelectronics, microprocessor control unit, and an energy source such asrechargeable batteries and/or an AC power conditioning circuit. Thesystem is implanted during a surgical procedure in which a centrifugalpump is placed in the patient's chest. An inflow conduit is pierced intothe left ventricle to supply blood to the pump. One end of an outflowconduit is mechanically fitted to the pump outlet and the other end issurgically attached to the patient's aorta by anastomosis. Apercutaneous cable connects to the pump, exits the patient through anincision, and connects to the external control unit.

Various aspects of the implantable pump are similar to those shown anddescribed in U.S. Pat. Nos. 4,528,485; 4,857,781; 5,229,693; 5,588,812;5,708,346; 5,917,297; 6,100,618; 6,222,290; 6,249,067; 6,268,675;6,355,998; 6,351,048; 6,365,996; 6,522,093; 7,972,122; and 8,686,674;and U.S. Pub. No. 2014/0205467 and 2012/0095281, the entire contents ofwhich patents and publications are incorporated herein by this referencefor all purposes.

The exemplary system utilizes an implantable pump with contactlessbearings for supporting the impeller. Contactless bearings (i.e.,levitation) provide a number of potential benefits. Because they reducerotational friction, theoretically they improve motor efficiency andreduce the risk of introducing particulates into the fluid. In oneexample, the impeller employs upper and lower plates having magneticmaterials (the terminology of upper and lower being arbitrary since thepump can be operated in any orientation). A stationary magnetic fieldfrom the upper side of the pump housing attracts the upper plate and arotating magnetic field from the lower side of the pump housing attractsthe lower plate. The forces cooperate so that the impeller rotates at alevitated position within the pumping chamber. Features (not shown) mayalso be formed in the walls of the pumping chamber to produce ahydrodynamic bearing wherein forces from the circulating fluid also tendto center the impeller. Hydrodynamic pressure grooves adapted to providesuch a hydrodynamic bearing are shown in U.S. Pat. No. 7,470,246, issuedDec. 30, 2008, titled “Centrifugal Blood Pump Apparatus,” which isincorporated herein for all purposes by reference.

The exemplary impeller has an optimal location within the pumpingchamber with a predetermined spacing from the chamber walls on eachside. Maintaining a proper spacing limits the shear stress and the flowstasis of the pump. A high shear stress can cause hemolysis of the blood(i.e., damage to cells). Flow stasis can cause thrombosis (i.e., bloodclotting).

With continued reference to FIG. 1, a patient is shown in fragmentaryfront elevational view. Surgically implanted either into the patient'sabdominal cavity or pericardium 11 is the pumping unit 12 of aventricular assist device. An inflow conduit (on the hidden side of unit12) pierces the apex of the heart to convey blood from the patient'sleft ventricle into pumping unit 12. An outflow conduit 13 conveys bloodfrom pumping unit 12 to the patient's ascending aorta. A percutaneouspower cable 14 extends from pumping unit 12 outwardly of the patient'sbody via an incision to a compact control unit 15 worn by patient 10.Control unit 15 is powered by a main battery pack 16 and/or an externalAC power supply, and an internal backup battery. Control unit 15includes a commutator circuit for driving a motor within pumping unit12.

In various embodiments, the commutator circuit and/or variouselectronics may be on the implanted side of the system. For example,various electronics may be positioned on-board the pump or in a separatehermetically sealed housing. Among the potential advantages ofimplanting electronics is the ability to control the pump even whencommunication is lost with the control unit 15 outside the body.

FIGS. 2 and 3 show exemplary centrifugal pumping devices in accordancewith various aspects of the invention. The exemplary pumping deviceincludes an impeller position controller to modify or select a positionfor the impeller. As will be described in further detail below, theimpeller is positioned with a fixed pump chamber such that movementcausing one pump gap to decrease generally causes the other pump gap toincrease, and vice versa.

FIG. 2 shows exemplary centrifugal pump unit 20 used in the system ofFIG. 1. The pump unit 20 includes an impeller 21 and a pump housinghaving upper and lower halves 22 a and 22 b. Impeller 21 is disposedwithin a pumping chamber 23 over a hub 24. Impeller 21 includes a firstplate or disc 25 and a second plate or disc 27 sandwiched over aplurality of vanes 26. Second disc 27 includes a plurality of embeddedmagnet segments 44 for interacting with a levitating magnetic fieldcreated by levitation magnet structure 34 disposed against housing 22 a.For achieving a small size, magnet structure 34 may comprise one or morepermanent magnet segments providing a symmetrical, static levitationmagnetic field around a 360° circumference. First disc 25 also containsembedded magnet segments 45 for magnetically coupling with a magneticfield from a stator assembly 35 disposed against housing 22 b. Housing22 a includes an inlet 28 for receiving blood from a patient's ventricleand distributing it to vanes 26. Impeller 21 is preferably circular andhas an outer circumferential edge 30. By rotatably driving impeller 21in a pumping direction 31, the blood received at an inner edge ofimpeller 21 is carried to outer circumferential 30 and enters a voluteregion 32 within pumping chamber 23 at an increased pressure. Thepressurized blood flows out from an outlet 33 formed by housing features33 a and 33 b. A flow-dividing guide wall 36 may be provided withinvolute region 32 to help stabilize the overall flow and the forcesacting on impeller 21.

FIG. 3 shows impeller 21 located in a balanced position. In theexemplary embodiment, the balanced position is at or near the center ofthe pump chamber. In the balanced position, the forces acting on theimpeller are generally balanced to stabilize the impeller. The balancedposition sometimes refers to the position the impeller naturallystabilizes or finds equilibrium during operation.

As one will understand from the description above, however, the balancedposition is not necessarily a specific, static location. Thehydrodynamic forces on the impeller will change as the rotational speedof the impeller changes. In turn, the magnetic attractive forces on theimpeller will change as the impeller moves closer to or away from themagnet structure 34 and stator assembly 35. Accordingly, the impellergenerally finds a new balanced position as the rotational speed changes.As will be described below, however, aspects of the invention aredirected to moving the impeller or changing the balanced position foreach given rotational speed. For example, the impeller position controlmechanisms to be described facilitate moving the impeller axially (up ordown) without changing the rotational speed and all other. This has theeffect of enabling movement of the impeller independent of rotor speed.An advantage of this technique is that rotor speed can be determined innormal course (e.g. by a physician based on the patient's physiologicalneeds) without concern for changing the impeller position. Conversely,the impeller position can be changed without affecting pumpingthroughput.

FIG. 3 shows impeller 21 located at or near a centered position whereindisc 27 is spaced from housing 22A by a gap 42 and impeller disc 25 isspaced from housing 22B by a gap 43. In the exemplary embodiment, thecenter position is chosen as the balanced or eccentric point to ensuresubstantially uniform flow through gap 42 and gap 43. During pumpoperation, the balanced position is maintained by the interaction of (a)attractive magnetic forces between permanent magnets 40 and 41 inlevitation magnet structure 34 with imbedded magnetic material 44 withinimpeller disc 27, (b) attractive magnetic forces between stator assembly35 and embedded magnet material 45 in impeller disc 25, and (c)hydrodynamic bearing forces exerted by the circulating fluid which maybe increased by forming hydrodynamic pressure grooves in housing 22 (notshown). By using permanent magnets in structure 34 a compact shape isrealized and potential failures associated with the complexities ofimplementing active levitation magnet control are avoided. To properlybalance impeller 21 at the centered position, however, and because otherforces acting on impeller 21 are not constant, an active positioningcontrol is still needed. In particular, the hydrodynamic forces actingon impeller 21 vary according to the rotational speed of impeller 21.Furthermore, the attractive force applied to impeller 21 by statorassembly 35 depends on the magnitude of the magnetic field and the angleby which the magnetic field leads the impellers magnetic field position.In one embodiment, the attractive force is created by a direct current(I_(d)) as will be described in more detail below.

In one embodiment, the impeller position is controlled using vectormotor control. Several structures and techniques for modifying impellerposition using vector motor control will now be described with referenceto FIGS. 3 to 7.

FIG. 4a shows the main structure of an exemplary centrifugal pump 50similar to that shown in FIG. 3. It is understood that other pumpconfigurations may be employed, including various combinations ofpermanent magnets, motor stator windings, and hydrodynamic bearings. Inthe exemplary embodiment, the rotor is formed as an impeller and drivenby a motor. The impeller is also levitated by the combined force F_(aΣ),which can be expressed as the following equation:F _(aΣ) =F _(hdb) +F _(pm) +F _(em)

Where,

F_(aΣ) is the combined force to levitate the impeller

F_(hdb) is the combination of hydrodynamic forces from the inlet sidebearing, the motor side bearing, or both

F_(pm) is the combination of permanent magnet attraction forces

F_(em) is the magnetic attraction force generated from the motor.

When the impeller is stabilized, F_(aΣ) should be equal to zero. UsuallyF_(em) can be controlled through the electronic system to adjust theimpeller position since all the others are the fixed configurations asthe passive mode. Therefore, the basic design concept of this inventionis to apply the motor vector control (FOC) to control the force F_(em)so that the impeller position can be adjusted while rotating only usingone set of motor coil and drive system. In such way, there is noadditional cost in the pump structure.

FIGS. 4a, 4b, and 4c illustrate a pump in accordance with variousaspects of the invention. FIG. 4a illustrates an exemplary pump 50 withan impeller 52 in a pump chamber. Pump 50 is configured similar to pump10 in FIG. 3. FIGS. 4b and 4c illustrate the same pump 50 in FIG. 4aexcept with the impeller moved down and up, respectively, in accordancewith the invention. In FIG. 4b , the impeller 52 is in a loweredposition such that Gap 2 is smaller and Gap 1 is commensurately wider.In this position the washout rate across Gap 1 is exponentially higherrelative to FIG. 4a . In FIG. 4c , the impeller 52 is in a raisedposition such that Gap 2 is wider and Gap 1 is commensurately narrower.In this position, the washout rate in Gap 2 is exponentially higherrelative to FIG. 4 a.

In various respects, the washout rate refers to the average washout rateduring a respective period of time. In various respects, the washoutrate refers to the peak washout rate. The respective period of time maybe any designated period of time, for example, the period during whichthe impeller is moved to a target position to increase the washout rate.

FIG. 5 illustrates an exemplary simplified method for controlling thepump. In step S1, the pump is operated with the impeller at a firstbalanced position. In step S2, a trigger is identified. In response, theimpeller is moved to a second position in step S3. Thereafter theimpeller eventually moves back to the first position.

The trigger may be temporal-based or event-based. In variousembodiments, the position control mechanism is configured to move theimpeller periodically and intermittently. In other words, the triggercan be the passage of a predetermined amount of time and/or based on aset frequency and cycle. In one example, the position control mechanismis configured to move the impeller for at least several seconds everyminute. In various embodiments, the position control mechanism isconfigured to move the impeller based on the impeller crossing a speedthreshold. The speed threshold may be a low speed threshold. The triggermay be based on a low speed threshold and time threshold. For example,the impeller may be moved after it spends more than a set amount of timeat a low speed. This may be beneficial because low speeds can lead totouchdown events, stasis, and other issues. Thus, the impeller may bemoved to ensure any particulates or thrombus are cleared from the gap.

In various embodiments, the pump is configured with at least a firstbalanced position with a narrow first gap and a second balanced positionwith a narrow second gap. As compared to FIG. 4a , FIG. 4b shows theimpeller defining a narrow Gap 2 and FIG. 4c shows the impeller defininga narrow Gap 1. Because the pump chamber dimensions are fixed and thetotal gap is likewise fixed, Gap 1 increases by the same distance thatGap 2 decreases. In FIG. 4c , the impeller has moved upward to a thirdbalanced position such that Gap 1 has decreased and Gap 2 has increasedby a commensurate amount.

The impeller may be controlled such that the impeller spendssubstantially equal amounts of time in the first and second balancedpositions. This may be useful where the pump is otherwise designed forthe impeller to normally be in a centered position, such as shown inFIG. 4a . In various embodiments, the amount of time the impeller spendsin each balanced position is inversely proportional to the gap size.This may be useful where the pump is otherwise designed to have unevengaps. In various embodiments, one of the gaps is identified as beingprone to stasis and the impeller spends more time in a position awayfrom the identified gap.

In various embodiments, the movement of the impeller is asynchronouswith the native heartbeat. In various embodiments, the movement of theimpeller is synchronous with the native heartbeat.

In various embodiments, a total blood gap under normal operatingconditions is 50 micrometers. In various embodiments, a total blood gapunder normal operating conditions is 100 micrometers. In variousembodiments, a total blood gap under normal operating conditions is 200micrometers. In various embodiments, a total blood gap under normaloperating conditions is 1000 micrometers. In various embodiments, atotal blood gap under normal operating conditions is 2000 micrometers.In various embodiments, the impeller is moved to a position to decreasea respective blood gap by about 20%, by about 30%, by about 40%, byabout 50%, by about 60%, by about 70%, by about 75%, by about 80%, or byabout 90%.

One will appreciate that FIGS. 4a, 4b, and 4c are illustrative only. Inpractice the impeller is freely suspended and not perfectly fixed in aposition. Because the exemplary pump suspends the impeller by balancinga combination of passive forces, the impeller actually exhibits amoderate amount of movement in practice. Indeed, the impeller will moveup and down depending on the rotational speed at least because of therelationship of rpms to hydrodynamic force. However, for a setrotational speed, the impeller is typically constrained with a definedenvelope of space which is referred to her as a “position” forsimplicity of explanation.

Mechanical or contact bearings exhibit little to no movement regardlessof the rotational speed and other factors. Nonetheless, contact bearingsdemonstrate some operational movement even if such movement requiresprecise instruments to be measured. Many types of blood pumps, forexample, utilize bearings which are washed and lubricated by an externalsource. Examples of a pump with a blood-immersed bearing are describedin U.S. Pub. No. 2012/0095281 and U.S. Pat. No. 5,588,812, incorporatedherein for all purposes by reference. In one example, a pump includesblood immersed contact bearings such as a ball-and-cup. In one example,the bearings are washed and/or lubricated by saline or infusate. The useof saline is a common scenario for percutaneous pumps because they havea fluidic connection to sources outside the body. Examples ofpercutaneous pumps with contact bearings are disclosed in U.S. Pat. Nos.7,393,181 and 8,535,211, incorporated herein by reference for allpurposes. As will be appreciated by one of skill in the art, contactbearings which are designed to have a fluid at least periodicallywashing between the contact surface will have some movement. Althoughthis movement is small relative to non-contact bearings (e.g. on theorder of 5, 10, 20, 100 or more times smaller), they are subject to somedegree of movement.

Conventional thinking is that a blood pump (e.g. left ventricular assistdevice) should be designed to maintain a stable impeller position andconsistent blood gaps across the device lifetime. In blood pumps, inparticular, movement of the impeller is often associated with hemolysisand other undesirable risks. There is a belief that decreasing a pumpgap creates a region of stasis which leads to thrombus and other adverseevents.

However, it has been found that adjusting the pump gap in a controlledand designed manner can actually improve performance and outcomes.Various aspects of the invention are directed to pumps configured toactively and purposefully modify the impeller position. In oneembodiment, the balanced position of the impeller is changed duringoperation.

There are several potential benefits to the technique described abovefor moving the impeller and alternating the pump gaps. One of thesepotential benefits is the ability to increase the peak washout flowvelocity. Another potential benefit is the ability to prevent or reducethe collection of ingested thrombus in narrow gaps without increasingthe total gap size. In turn, pump efficiency and performance are notcompromised. Existing solutions (e.g. stable-gap hydrodynamic bearingdesigns) rely on the native heart to change the blood flow pattern inthe narrow gap areas (such as systole/diastole). The inventive techniqueis advantageous because it actively changes the flow patternindependently of the native heart function, impeller speed, etc. Also,many heart failure patients have weakened native hearts withinsufficient pulsatility to actually wash out the bearing gaps. With theinventive technique, the combination of the external pressure change andinternal geometry change (rotor position change) will minimize the bloodflow stasis which causes pump thrombosis.

FIGS. 6 and 7 illustrate an exemplary method for controlling voltagesapplied to a stator in order to provide a desired rotation for apermanent magnet rotor (e.g. the impeller) 52 using a field orientedcontrol (FOC) algorithm, which is also known as vector control. It isknown in FOC that the stator magnetic field should generally lead theimpeller position by 90° for maximum torque efficiency. The magnitude ofthe attractive force on the impeller is proportional to the magnitude ofthe phase currents in the stator. Phase current is adjusted by the FOCalgorithm according to torque demands for the pump.

At any particular combination of the (1) magnitude of the phase currentand (2) the speed of the impeller, modifying the I_(d) current forgenerating the phase voltages can change the attractive force generatedby the stator thereby affecting the impeller balance. In turn, theimpeller moves until it settles at a new balanced position where thehydrodynamic forces and magnetic forces are balanced. In this manner,the impeller can be moved simply by adjustments to the motor controlsignal.

FIGS. 4b, 4c , 5, 6, and 7 illustrate an exemplary system in accordancewith aspects of the invention. FIG. 6 is a schematic diagram of pumpcontrol system with the proposed impeller position control. Based on theprinciple of motor vector control, the torque current that is usuallycalled quadrature current (I_(q) current) and stator coil flux currentthat is called direct current (I_(d) current) can be decoupled andcontrolled independently. The quadrature current I_(q) current is usedto control the impeller rotational speed. The direct I_(d) currentcontrols the magnetic flux of electromagnetic coils which creates aresulting attractive force on the impeller.

In accordance with the invention, I_(d) current is utilized to controlthe impeller position by enhancing or weakening the magnetic fluxbetween impeller (rotor) and motor stator coils to adjust the attractionforce F_(em). This in turn changes the impeller position (shown in FIG.7). In various embodiments, the attractive force is created by adjustingthe phase angle using FOC.

In one embodiment, the impeller position control technique isimplemented as an open loop control without impeller position sensors.In one embodiment, impeller position control technique is implemented asa closed loop control with impeller position sensors.

In order to ensure proper positioning, active monitoring and control ofthe impeller position has been employed in the exemplary embodiment byadjusting the stationary magnetic field. However, position sensors andan adjustable magnetic source occupy a significant amount of space andadd to the complexity of a system. Accordingly, the use of sensors maydepend on the design requirements. Suitable sensors may include, but arenot limited to, Hall-effect sensors, variable reluctance sensors, andaccelerometers.

In one embodiment using the open loop control, the impeller iscontrolled by periodically alternating the position from one side toanother (e.g. from inlet side to motor side) by modulating the I_(d)current as shown in FIG. 7. In this manner, the side gaps (Gap 1 and Gap2) as shown in FIGS. 4b and 4c can be increased or decreased.

With continued reference to FIGS. 6 and 7, the position controltechnique can be implemented into the hardware and/or software of thesystem. By example, the controller may employ FOC to supply a multiphasevoltage signal to the stator assembly 53. The exemplary stator assemblyis a three-phase stator. Individual phases a, b, and c and currentsI_(a), I_(b), and I_(c) may be driven by an H-bridge inverterfunctioning as a commutation circuit driven by a pulse width modulator(PWM) circuit. An optional current sensing circuit associated with theinverter measure instantaneous phase current in at least two phasesproviding current signals designated I_(a) and I_(b). A currentcalculating block receives the two measured currents and calculates acurrent I_(c) corresponding to the third phase. The measured currentsare input to Vector Control (FOC) block 54 and to a current observerblock (not shown) which estimates the position and speed of theimpeller. The impeller position and speed are input to the FOC blockfrom speed control block 55 and position control block 56. A targetspeed or revolutions per minute (rpm) for operating the pump is providedby a conventional physiological monitor to FOC block 54. The target rpmmay be set by a medical caregiver or determined according to analgorithm based on various patient parameters such heartbeat,physiological needs, suction detection, and the like. FOC block 54 anddrive electronics 57 generate commanded voltage output values Va, Vb,and Vc. The Va, Vb, and Vc commands may also be coupled to the observerblock for use in detecting speed and position.

The exemplary system differs from conventional configurations inasmuchas the FOC block and electronics are configured to alter the fieldoriented control algorithm so that the I_(d) current can be variedindependently to generated the required attractive force. The exemplarysystem potentially sacrifices such efficiency in return for otherbenefits. Among the benefits of the exemplary system is the ability toindependently control the impeller position.

FIG. 8 illustrates a method of moving the impeller using a FOCalgorithm. In one embodiment, the invention proceeds according to amethod as shown in FIG. 8 which highlights a portion of the impellerposition control with the field oriented control algorithm. Thus, instep 65 the phase currents are measured. Based on the measured phasecurrents, the current speed and rotor angle are estimated in step 66based on the rotor angle determined in step 66. The phase currents aretransformed into a two-axis coordinate system to generate quadraturecurrent (called I_(q) current) and direct current (called I_(d) current)values in a rotating reference frame in step 67. Quadrature current isused to control the torque to rotate the impeller and direct current isused to control the attraction force between rotor and stator to controlthe impeller position. In step 68, the next quadrature voltage isdetermined by the quadrature current error between the quadraturecurrent transformed from step 67 and the required current for impellerrotation. In step 69, the next direct voltage is determined by thedirect current error between the direct current transformed from step 67and the required current for the attraction force alternation to controlthe impeller position. In step 70, the quadrature and direct voltage aretransformed back to the stationary reference frame in order to providethe multiphase voltage commands which are output to the PWM circuit.

FIG. 9 is a flowchart showing another method of operating a rotarymachine in accordance with the invention. The method includes operatingthe pump to rotate the impeller by applying a rotating magnetic field instep S10. During operation the impeller is levitated and positioned at abalanced position (P₁) by a balancing of forces. As described above, inan exemplary embodiment the impeller is levitated by the combination ofhydrodynamic forces F₁ and other bearing forces F₂ (e.g. statorattractive force, passive magnetic forces, and/or bulk forces likegravity) in steps S11 and S12. Next, at least one of the forces, F₂, ismodified to place the impeller out of balance in step S14. The impellermoves to a new position, P₂, where the forces are once again balanced instep S15.

Turning to FIG. 10, in one embodiment, the impeller position controltechnique is used to facilitate start-up of the pump. In step S20, theexemplary pump is configured so the impeller rests against the inletside (top of the housing) when the impeller is not rotating. In atypical pump with hydrodynamic forces alone, or in combination withmagnetic forces, the impeller is levitated away from the wall as itrotates. The blood entrained in the gap between the impeller and thehousing creates hydrodynamic pressure; however, the impeller must berotating at a sufficient speed to create the hydrodynamic pressure.Until the minimum speed is met, the impeller rubs against the housingwall. In the exemplary pump, by contrast, the impeller is pulled awayfrom the wall prior to, or just after, rotation begins therebyeliminating the deleterious effects of friction. The impeller is pulledaway from the wall by applying a force, F, as described above in stepS21. For example, the commutation angle may be modified to exert anattractive force. Referring to FIG. 7b , by example, the pump can beconfigured so the impeller rests at the inlet side. By applying anattractive force to the motor side the impeller moves down from the topwall. In step S22, the regular start-up sequence is initiated after theimpeller is removed from the wall.

FIG. 11 shows a rotary machine in accordance with another embodimentmaking use of electromagnets. Pump 100 in FIG. 11 is similar in variousrespects to pump 10 in FIG. 3. In the exemplary embodiment, however,pump 100 includes an active electromagnetic (EM) system 101. The EMforce generated by electromagnets is used primarily or adjunctively tomove the impeller. Exemplary electromagnets 101 comprise iron cores andwindings. The EM force is modified in a conventional manner by changingthe current applied to the windings. The application of the EM forcecauses the impeller to move to position P_(E2). One will appreciate thatthe EM force can overpower hydrodynamic and passive magnetic forcespresent in the system. Accordingly, the EM structure must dimensionedand configured to apply a relatively balanced force. An advantage ofusing electromagnets over the existing stator assembly is that there isrelatively greater positional control over the impeller. By contrast, asdescribed above, the phase currents typically cannot be used as theprimary variable to adjust the axial attractive force on the impeller. Adisadvantage of this embodiment is the need to provide an entirelyseparate EM system. This may not be an issue with large industrialrotary machines, but many types of motors have restrictive form factors.For example, implanted pumps must be relatively small in order toaddress a wider patient population.

FIGS. 12 and 13 illustrate another implantable pump in accordance withthe invention. Pump 200 is similar in various respects to pumps 10 and100 described above except pump 200 is an axial flow pump. Blood flowsfrom in through inlet 201 and out through outlet 202 in a generallylinear, axial direction. Pump 200 includes an impeller 210 having bladesfor moving blood through the pump housing and imparting kinetic energyin the fluid.

Impeller 210 is fixed within the housing by ball-and-cup bearings 212and 214. The ball-and-cup bearings are closely toleranced and generallyfix the impeller in a specific position. However, the exemplary bearingsare lubricated and washed by the blood flow around the impeller.Accordingly, there is some fluid between the ball and cup surfaces.

Torque is applied to the impeller by a stator assembly 205. The statorassembly 205 includes windings and is driven using a FOC algorithm in asimilar manner to the stator assemblies described above. In practice,the impeller position is adjusted proceeding according to the methodshown in FIGS. 5 and/or 9. Using the FOC technique described above theimpeller is rotated in the pump housing. At a desired time the I_(d)current is modulated to adjust the attractive force on the impeller inthe axial direction. As long as the attractive force is sufficient tosqueeze blood out from a respective bearing gap, the impeller will moveaxially towards inlet 201 or outlet 202. The bearing gaps of pump 200are relatively small compared to Gap 1 and Gap 2 of pump 50 in FIG. 4.However, even relatively small impeller movement may be beneficial toenable control of the bearing gaps.

The method of adjusting the pump gaps to increase the washout rate maybe particularly beneficial in pump designs with mechanical bearings. Therelatively small gaps in the bearings mean that there is very littlefluid flow and thus a higher risk of thrombus. The greater friction alsocan contribute to greater thrombus risk. Accordingly, the ability toincrease the gap between the ball and the cup, even on a small scale,can lead to significant improvements in outcomes.

FIG. 14 illustrates another pump 300 similar to pump 200. Pump 300includes an impeller fixed between two mechanical bearings 212 and 214.Pump 300 is slightly different than pump 200 because the outlet extendsat an angle from the inlet. Pump 300 is configured in a relativelycompact design compared to pump 200 including a relatively smallerstator assembly; however, the same general principles can be applied tocontrol the motor and adjust the impeller position.

FIG. 15 is a cross-sectional view of another pump 400 similar to pumps200 and 300, except pump 400 is an axial flow pump with non-contactbearings. Pump 400 includes a pump housing having an inlet 401 andoutlet 402. An impeller 411 is positioned within a pump chamber forimparting flow to the blood fluid within the housing. The impeller isentirely formed of a magnetic material which is driven by interactionwith a stator assembly 405.

Impeller 411 is stabilized in the pump chamber by a combination ofhydrodynamic and passive magnetic forces. Impeller 411, which is amagnetic material, interacts with the magnetic material in statorassembly 405 to provide an axial centering force. A pump ring 452 with achamfer surface is positioned at the leading end of the impeller tocreate hydrodynamic stabilization forces in the axial direction (left toright) and radial direction (up and down on page). A permanent magnetring 450 is provided at the trailing edge of the impeller is orientedwith a north pole facing a north pole of the impeller. This arrangementcreates an axial bias force to push the impeller against the pump ring452. The magnet ring 450 also provides a radially centering force.Finally, the impeller includes deep hydrodynamic grooves to generate ahydrodynamic pressure force against the inner walls of the pump chamberfor radial stabilization.

In operation, the impeller remains stable in the axial and radialdirections. There may be some axial movement as the rotational speed ofthe impeller changes or as a result of other forces (e.g. the nativepulse), but generally the impeller remains centered below the statorassembly.

Using the FOC control technique described above, the attractive force ofthe stator assembly 405 on impeller 411 can be modified. In oneembodiment, pump 400 is configured so impeller is eccentric whencentered below the stator assembly 405. In this example, increasing theattractive force amounts to an increase in the axial stiffness to resistaxial movement. In one embodiment, the attractive force is modified toactually move impeller 411 axially. For example, the impeller can bemoved closer to pump ring 452 to squeeze blood out of the gap betweenimpeller 411 and a surface of ring 452. The impeller may also be movedaway from ring 452 to increase the blood gap therebetween. In thismanner, the impeller position control technique adds an element ofactive position control otherwise not possible with the passive bearingconfiguration of pump 400.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the Claims appended hereto and theirequivalents.

What is claimed is:
 1. A blood pump comprising: a stator for applyingforce to an impeller; a sensing circuit for determining an axialposition of the impeller; and a position control mechanism for movingthe impeller in an axial direction independent of a rotational speed ofthe impeller by controlling, based on the axial position of theimpeller, a field oriented control device which provides current to thestator.
 2. The blood pump of claim 1, wherein: the pump is ancentrifugal pump.
 3. The blood pump of claim 1, wherein: the pump is anaxial flow pump.
 4. The blood pump of claim 1, wherein: the positioncontrol mechanism is further for moving the impeller in the axialdirection between a first eccentric position and a second eccentricposition.
 5. The blood pump of claim 1, wherein: the position controlmechanism is further for moving the impeller in the axial direction upona trigger event occurring.
 6. The blood pump of claim 5, wherein: thetrigger event comprises the impeller rotating below a predefined speedfor a predefined length of time.
 7. The blood pump of claim 1, wherein:the position control mechanism is further for moving the impeller in theaxial direction between a first position and a second position; and theposition control mechanism is configured to maintain the impeller in thefirst position and the second position for substantially equal lengthsof time.
 8. The blood pump of claim 1, wherein: the position controlmechanism is further for moving the impeller in the axial directionbetween a first position and a second position, wherein: in the firstposition there is a first gap between the impeller and a first wall of apump chamber of the pump; in the second position there is a second gapbetween the impeller and a second wall of the pump chamber of the pump;and the first gap is smaller than the second gap; and the positioncontrol mechanism is configured to maintain the impeller in the firstposition for longer than in the second position.
 9. The blood pump ofclaim 1, wherein the sensing circuit comprises: a position sensorconfigured to determine the axial position of the impeller.
 10. Theblood pump of claim 1, wherein: the sensing circuit is configured todetermine the axial position of the impeller based on at least onemeasured current associated with the stator.
 11. A method forcontrolling a pump comprising: causing a stator to apply a rotationalforce to an impeller; determining an axial position of the impeller; andcausing, with a field oriented control device, the impeller to be movedin an axial direction based on the axial position of the impeller andindependent of a rotational speed of the impeller.
 12. The method forcontrolling a pump of claim 11, wherein causing the impeller to be movedcomprises: causing the impeller to be moved between a first eccentricposition and a second eccentric position.
 13. The method for controllinga pump of claim 11, wherein causing the impeller to be moved comprises:causing the impeller to be moved upon a trigger condition occurring. 14.The method for controlling a pump of claim 11, wherein causing theimpeller to be moved comprises: causing the impeller to be moved in theaxial direction between a first position and a second position; andmaintaining the impeller in the first position and the second positionfor substantially equal lengths of time.
 15. The method for controllinga pump of claim 11, wherein causing the impeller to be moved comprises:causing the impeller to be moved in the axial direction between a firstposition and a second position, wherein: in the first position there isa first gap between the impeller and a first wall of a pump chamber ofthe pump; in the second position there is a second gap between theimpeller and a second wall of the pump chamber of the pump; and thefirst gap is smaller than the second gap; and maintaining the impellerin the first position for longer than in the second position.
 16. Anon-transitory machine readable medium having instructions thereon forcontrolling a pump, wherein the instructions, when executed by at leastone processor, cause steps to be performed comprising: causing a statorto apply a rotational force to an impeller; determining an axialposition of the impeller; and causing a field oriented control device tomove the impeller axially based on the axial position of the impellerand independent of a rotational speed of the impeller.
 17. Thenon-transitory machine readable medium of claim 16, wherein moving theimpeller axially comprises: moving the impeller between a firsteccentric position and a second eccentric position.
 18. Thenon-transitory machine readable medium of claim 16, wherein causing thefield oriented control device to move the impeller comprises: causingthe field oriented control device to move the impeller upon a triggercondition occurring.
 19. The non-transitory machine readable medium ofclaim 16, wherein determining the axial position of the impellercomprises: determining at least one measured current associated with thestator; and determining the axial position of the impeller based on theat least one measured current.